My (32F) sister (27F) wants to marry my childhood friend/high school sweetheart but he (31M) slept with our estranged mom and my sister does not know, I think.

I grew up with my three sisters and mom - our father separated himself from our mom when our youngest sister was born. Our neighbor was also a single mom and had two sons. Me and her son David were very close growing up and dated all through high school. Growing up my mom had several set backs and unfortunate events which slowly chipped away at her mental health. In addition, during my senior year of high school and freshman year of college I rebelled against everyone and everything - especially my mom. I completely destroyed my relationship with David and my mom. I was a horrible daughter and even worse girlfriend to David. I lied, cheated, and manipulated both of them to get my way. David was amazing - no matter what I did he put up with it and supported me. My mom could see through all my bullshit and would constantly call me out on it. Eventually things came to a crash.
My mom came home and I was fooling around with another guy in my room. She barged in and refused to leave until the guy left - by this point I was heavy into drinking and was sleeping around - shortly after my mom barged in David came walking into my room. David knew our relationship was not good but he didn't know I was cheating on him. I still remember the look on his face - he sat muted in our living room couch as I tried to explain what happened and how much it wasn't my fault; nothing but lies. A few days later I ran away from home - I spent the next few months crashing at friends houses. I eventually got a job and rented my own place - got sober and met my now husband. We have two kids and live in Spain - he is in the Army and is stationed here.
David and his family were always very close with my family since we lived next door and basically grew up together. My mom always favored David - he really was/is perfect. Shortly after I ran away my mom decided to purse David - David himself went off the deep end and started drinking and partying. One thing lead to another and David and my mom end up sleeping together. The only reason why I found out is because my youngest sister caught them - I then confronted my mom over a phone call and she admitted it. She said it was a one time thing and would never happen again. By this point I didn't care and had hurt both my mom and David so much I just dropped the whole thing. I was already married and pregnant with my first. My two sisters had moved out as soon as my youngest sister turned 18. My mom lives on her own and moves around a lot. She has issues with anxiety and sometimes gets into states of depression.
My sisters and I are not close - we speak a few times and year. Around 2018 my youngest sister mentioned that David and my younger sister Ana were spending a lot of time together. Last week I got a call from Ana letting me know that her and David had discussed getting engaged and married by the end of the year. I haven't spoke to David in years. I called my youngest sister and asked if Ana knew about our mom and David sleeping together - she told me she never told anyone but me and was not planning on telling anyone else.

Is it even my place to say anything? I don't know if David told her?
I have kept my distance from my family since I have hurt them so much and now I feel if I speak up I could cause more damage.

TL;DR - Dated my childhood friend David during high school/college - cheated on him and was a terrible person/girlfriend. Lied and manipulated my family and mom. Took advantage of my moms mental health issues. Ran away got my act together, got married, had kids, moved to Spain. During that time David and my mom had sex. Years later David and my younger sister Ana became very close and are dating. They want to get engaged and married by the end of the year. I don't think Ana knows David and my mom had sex. At this point I think myself, Ana, and my mom have had sex with David.

Submitted July 25, 2019 at 12:15AM by waytogomom https://ift.tt/2yffoQs